Sleeping hours: a relatively protected period for impending myocardial infarction

We have retrospectively investigated the time of the onset of acute myocardial infarction in 2046 patients admitted to six coronary care units in a two-year period. A significantly reduced number of patients (P less than 0.01) showed the beginning of acute myocardial infarction during the 0 to 6 a.m. period, while, during the remaining periods, no difference in frequency distribution was observed. Our results suggest that an impending myocardial infarction is more likely to occur at certain times of the day than others, suggesting a period of relative protection from onset.